Abstract

Uncertainty in climate sensitivity is a fundamental problem for projections of the future climate. Climate sensitivity is defined as the equilibrium response of global-mean surface air temperature to a doubling of the atmospheric CO2 concentration from the preindustrial level (. 280 ppm). In spite of various efforts to estimate its value, climate sensitivity is still not well constrained (IPCC, 2007, pp.718-727 and pp.798-799; Gerard and Baker, 2007), posing a difficulty to informing climate change policy. Here we show that the climate sensitivity is in fact even more uncertain than has been found by earlier studies (Andronova and Schlesinger, 2001; Gregory et al., 2002; Knutti et al., 2002; Forest et al., 2006; Hegerl et al., 2006). Our results suggest that uncertainty in historical radiative forcing has not been sufficiently considered and that including a carbon cycle feedback, which in principle offers an additional constraint on climate sensitivity, does not reduce the uncertainty in climate sensitivity due to the poor knowledge of the global carbon budget before the year 1850.